1. Field of the Invention
The present invention relates to a solar cell, and more particularly, it relates to a solar cell utilizing an internal electric field formed with a pn junction or a pin junction, and a process for producing the same.
2. Description of the Related Art
Studies for utilizing the solar energy as environmentally thoughtful energy have been earnestly made. In particular, such a solid state device has been earnestly studied that converts the solar energy directly to electric energy using an inorganic semiconductor, owing to the high photoelectric conversion efficiency thereof, and solar cells utilizing single crystal, polycrystalline or amorphous silicon have been put into practical use. The solar cell utilizing an inorganic semiconductor attains a photoelectric conversion efficiency (energy conversion efficiency, i.e., a ratio of generated electric energy to incident light energy) reaching, for example, about 15% in the case using silicon.
The semiconductor layer, which is an important constitutional element of the solar cell, has a semiconductor junction, such as a pn junction (i.e., a p-type semiconductor substance and an n-type semiconductor substance are directly joined to each other) and a pin junction (i.e., a p-type semiconductor substance and an n-type semiconductor substance are joined with an intermediate layer intervening therebetween). The semiconductor junction is formed by sequentially accumulating semiconductor layers having different conductivity types, or doping a semiconductor layer by injecting a dopant having a conductivity type different from the semiconductor layer into the semiconductor layer by the ion injection method, or by diffusing the dopant into the semiconductor layer by the thermal diffusion method.
However, the production process of the solar cell using an inorganic thin film semiconductor requires a special equipment to raise the production cost of the solar cell, and furthermore, the process uses toxic raw material gases. Therefore, the process cannot be environmentally thoughtful.
In a solar cell using an organic semiconductor, on the other hand, various studies for practical application have been made owing to the excellent characteristics, i.e., an organic semiconductor has wide variety and low toxicity and is good in workability to enable reduction in cost with mass production. However, a solar cell using an organic semiconductor has not yet been put into practical application because of the low photoelectric conversion efficiency thereof. The reason why the solar cell using an organic semiconductor has a low photoelectric conversion efficiency includes the low photocarrier forming capability of the organic semiconductor, the narrow photocarrier forming region around the junction as compared to an inorganic semiconductor, and the low capability of transporting the carriers thus formed. As a result, only a small electric current is formed to provide low photoelectric conversion efficiency.
In recent years, carbon nanotubes, which have an extremely minute size and peculiar characteristics in semiconductor or conductor property and other electric and physical properties, have been studied as raw materials for various purposes. Various electric and electronic devices, such as a transistor, using carbon nanotubes have been developed. However, these electric or electronic devices use carbon nanotubes for miniaturizing them as disclosed in JP-A-2004-179564, or for improving the eleetroconductivity of the devices as disclosed in J. App. Phys., vol. 93, p. 1764 (2003). Accordingly, they not only contains no application to a solar cell, which requires a certain area for receiving light, but also contains no concept of the application to a solar cell.